The present invention relates to a developing unit incorporated in an image forming apparatus and having a fresh toner container and a waste toner container which adjoin each other.
It is a common practice with a copier, facsimile transceiver, laser printer or similar image forming apparatus to electrostatically form a latent image on a photoconductive element by an electrophotographic process, develop the latent image to produce a toner image by a developing unit, and then transfer the toner image to a recording medium. The toner remaining on the photoconductive element after the image transfer is removed by a cleaning device.
The developing unit may be provided with a waste toner container and a fresh toner container adjoining each other with the intermediary of a wall, as disclosed in, for example, Japanese Patent Laid-Open Publication No. 74033/1974. In this type of developing device, the toner removed from the photoconductive element, i.e., waste toner is collected in the waste toner container, while a fresh toner to be fed to the developing unit is stored in the fresh toner container. As soon as the fresh toner container runs out of the fresh toner, it is removed from the unit together with the waste toner container and discarded. To enhance the efficient use of toner and the life of the unit, the waste toner collected in the waste toner container is again fed to a toner hopper to be reused for development.
However, reusing the waste toner is not desirable from the developing characteristic standpoint since it has a different developing characteristic from the fresh toner, i.e., it is usually constituted by the toner left in the image area and background of the photoconductive element without being transferred to a recording medium. This part of the toner carries an opposite charge or an unstable charge thereon. In light of this, the entire amount of waste toner may be collected in the waste toner container adjoining the fresh toner container. This, however, has a problem that the amount of waste toner to be produced depends on the ambient conditions and the kind of documents and, is, therefore, almost impossible to estimate beforehand. While the waste toner container may be provided with a sufficient volume, i.e., a volume which would accommodate the entire fresh toner, such a waste toner container is impractical since the space available in the developing unit is limited. In practice, therefore, the amount of waste toner to be produced is estimated by multiplying the amount of fresh toner by an experiential ratio and adding some margin to the product. As a result, when the actual ambient conditions are different from estimated ones, it is likely that the waste toner fills up the waste toner container and, in the worst case, flows out of such a container to smear the interior of the apparatus.